Adenosine A2a Receptors
Adenosine is a purine nucleotide produced by all metabolically active cells within the body. Adenosine exerts its effects via four subtypes of cell-surface receptors (A1, A2a, A2b and A3), which belong to the G protein coupled receptor superfamily (Stiles, G. L. Journal of Biological Chemistry, 1992, 267, 6451). A1 and A3 couple to inhibitory G protein, while A2a and A2b couple to stimulatory G protein. A2a receptors are mainly found in the brain, both in neurons and glial cells (highest level in the striatum and nucleus accumbens, moderate to high level in olfactory tubercle, hypothalamus, and hippocampus etc. regions) (Rosin, D. L.; Robeva, A.; Woodard, R. L.; Guyenet, P. G.; Linden, J. Journal of Comparative Neurology , 1998, 401, 163).
In peripheral tissues, A2a receptors are found in platelets, neutrophils, vascular smooth muscle and endothelium (Gessi, S.; Varani, K.; Merighi, S.; Ongini, E.; Borea, P. A. British Journal of Pharmacology, 2000, 129, 2). The striatum is the main brain region for the regulation of motor activity, particularly through its innervation from dopaminergic neurons originating in the substantia nigra. The striatum is the major target of the dopaminergic neuron degeneration in patients with Parkinson's Disease (PD). Within the striatum, A2a receptors are co-localized with dopamine D2 receptors, suggesting an important site of for the integration of adenosine and dopamine signaling in the brain (Fink, J. S.; Weaver, D. R.; Rivkees, S. A.; Peterfreund, R. A.; Pollack, A. E.; Adler, E. M.; Reppert, S. M. Brain Research Molecular Brain Research, 1992, 14, 186).
Neurochemical studies have-shown that activation of A2a receptors reduces the binding affinity of D2 agonist to their receptors. This D2R and A2aR receptor-receptor interaction has been demonstrated in striatal membrane preparations of rats (Ferre, S.; von Euler, G.; Johansson, B.; Fredholm, B. B.; Fuxe, K. Proceedings of the National Academy of Sciences of the United States of America, 1991, 88, 7238) as well as in fibroblast cell lines after transfected with A2aR and D2R cDNAs (Salim, H.; Ferre, S.; Dalal, A.; Peterfreund, R. A.; Fuxe, K.; Vincent, J. D.; Lledo, P. M. Journal of Neurochemistry, 2000, 74, 432). In vivo, pharmacological blockade of A2a receptors using A2a antagonist leads to beneficial effects in dopaminergic neurotoxin MPTP(1-methyl-4-pheny-l,2,3,6-tetrahydropyridine)-induced PD in various species, including mice, rats, and monkeys (Ikeda, K.; Kurokawa, M.; Aoyama, S.; Kuwana, Y. Journal of Neurochemistry, 2002, 80, 262). Furthermore, A2a knockout mice with genetic blockade of A2a function have been found to be less sensitive to motor impairment and neurochemical changes when they were exposed to neurotoxin MPTP (Chen, J. F.; Xu, K,; Petzer, J. P.; Staal, R.; Xu, Y. H.; Beilstein, M.; Sonsalla, P. K.; Castagnoli, K.; Castagnoli, N., Jr.; Schwarzschild, M. A. Journal of Neuroscience, 2001, 21, RC143).
In humans, the adenosine receptor antagonist theophylline has been found to produce beneficial effects in PD patients (Mally, J.; Stone, T. W. Journal of the Neurological Sciences, 1995, 132, 129). Consistently, recent epidemiological study has shown that high caffeine consumption makes people less likely to develop PD (Ascherio, A.; Zhang, S. M.; Hernan, M. A.; Kawachi, I.; Colditz, G. A.; Sp izer, F. E.; Willett, W. C. Annals of Neurology, 2001, 50, 56). In summary, adenosine A2a receptor blockers may provide a new class of antiparkinsonian agents (Impagnatiello, F.; Bastia, E.; Ongini, E.; Monopoli, A. Emerging Therapeutic Targets, 2000, 4, 635).
Phosphodiesterase Inhibitors
There are eleven known families of phosphodiesterases (PDE) widely distributed in many cell types and tissues. In their nomenclature, the number indicating the family is followed by a capital letter that indicates a distinct gene. A PDE inhibitor increases the concentration of CAMP in tissue cells, and hence, is useful in the prophylaxis or treatment of various diseases caused by the decrease in cAMP level which is induced by the abnormal metabolism of CAMP. These diseases include conditions such as hypersensitivity, allergy, arthritis, asthma, bee sting, animal bite, bronchospasm, dysmenorrhea, esophageal spasm, glaucoma, premature labor, a urinary tract disorder, inflammatory bowel disease, stroke, erectile dysfunction, HIV/AIDS, cardiovascular disease, gastrointestinal motility disorder, and psoriasis.
Among known phosphodiesterases today, PDE1 family are activated by calcium-calmodulin; its members include PDE1A and PDE1B, which preferentially hydrolyze cGMP, and PDE1C which exhibits a high affinity for both CAMP and cGMP. PDE2 family is characterized as being specifically stimulated by cGMP. PDE2A is specifically inhibited by erythro-9(2-hydroxy-3-nonyl)adenine (EHNA). Enzymes in the PDE3 family (e.g. PDE3A, PDE3B) are specifically inhibited by cGMP. PDE4 (e.g. PDE4A, PDE4B, PDE4C, PDE4D) is a cAMP specific PDE present in T-cells, which is involved in inflammatory responses. A PDE3 and/or PDE4 inhibitor would be predicted to have utility in the following disorders: autoimmune disorders (e.g. arthritis), inflammatory bowel disease, bronchial disorders (e.g. asthma), HIV/AIDS, and psoriasis. A PDE5 (e.g. PDE5A) inhibitor would be useful for the treatment of the following disorders: cardiovascular disease and erectile dysfunction. The photoreceptor PDE6 (e.g. PDE6A, PDE6B, PDE6C) enzymes specifically hydrolyze cGMP. PDE8 family exhibits high affinity for hydrolysis of both cAMP and cGMP but relatively low sensitivity to enzyme inhibitors specific for other PDE families.
Phosphodiesterase 7 (PDE7A, PDE7B) is a cyclic nucleotide phosphodiesterase that is specific for cyclic adenosine monophosphate (cAMP). PDE7 catalyzes the conversion of cAMP to adenosine monophosphate (AMP) by hydrolyzing the 3′-phosphodiester bond of cAMP. By regulating this conversion, PDE7 allows for non-uniform intracellular distribution of cAMP and thus controls the activation of distinct kinase signalling pathways. PDE7A is primarily expressed in T-cells, and it has been shown that induction of PDE7A is required for T-cell activation (Li, L.; Yee, C.; Beavo, J. A. Science 1999, 283, 848). Since PDE7A activation is necessary for T-cell activation, small molecule inhibitors of PDE7 would be useful as immunosuppressants. An inhibitor of PDE7A would be predicted to have immunosuppressive effects with utility in therapeutic areas such as organ transplantation, autoimmune disorders (e.g. arthritis), HIV/AIDS, inflammatory bowel disease, asthma, allergies and psoriasis.
Few potent inhibitors of PDE7 have been reported. Most inhibitors of other phosphodiesterases have IC50's for PDE7 in the 100 μM range. Recently, Martinez, et a/. (J. Med. Chem. 2000, 43, 683) reported a series of PDE7 inhibitors, among which the two best compounds have PDE7 IC50's of 8 and 13 μM. However, these compounds were only 2-3 times selective for PDE7 over PDE4 and PDE3.
Finally the following compounds have been disclosed, and some of them are reported to show antimicrobial activity against strains such as Plasmodium falciparum, Candida albicans and Staphylococcus aureus (Gorlitzer, K.; Herbig, S.; Walter, R. D. Pharmazie 1997, 504): 